What battery protection fuse for new LifePO4 install?

[andrewAB]

Hi,
Trying to decide on what if any external battery fuse to use on a new battery.

I'm looking to replace my 7 years old pair of Trojan T-105s (220AH @12V originally) with a EEL DIY kit 12.8V and 280Ah CATL cells.

The EEL kit comes with an internal 250 Amp CNL fuse and 2.5K Amp interrupt rating.
MRBF terminal fuse have 10K Amp interrupt rating.
The best to use would be Class T with 20K Amp interrupt rating.

Reading the specs I've found online BlueSeas 250A Class-T fuse would blow at 1000A in 0.1 seconds at a current below its interrupt current rating , whereas a Littlefuse 250A CNL would take 2500A in 0.1 seconds and could theoretically arc if the battery terminals were short circuited.

The Blueseas MRBF would blow with 1375A in 0.1s and well under it's interrupt current rating. This seems safe enough to me and easy to install. Any issues with using this?

If it is a decided a Clsss T is required how would I connect the Class-T fuse holder to the battery? A 70mm2 (485A) 30cm cable with M10 lugs is what I use to my Smartshunt on the ground, so similar on the 12V main?

BTW following are link to the EEL battery kit and cells they can provide in Europe and a summary of the blow time vs current I found from online specs.
Thanks
Andrew


https://www.eelbattery.com/12v-life...e-bms-set-eu-warehouse-shipping-p5733618.html
https://www.eelbattery.com/copy-of-...or-solar-energy-diy-eu-shipping-p5842165.html


Blueseas Class-T trip delay
20K Amp interrupt rating, 125V, extremely fast acting. Would blow with 1000A in 0.1 seconds.

Littlefuse CNL 250
2.5K Amp interrupt rating, 32V, fast acting. Would blow with 2500A in 0.1 seconds -> at interrupt rating current

Blue Seas MRBF 250A 10K Amp interrupt rating. Would blow with 1375A in 0.1 seconds

Time Class-T CNL MRBF
-------------------------------------------------
0.01 5X rated 25X
0.1 4X 10X 5.5X
1 3X 3.2X 3X
10 2.5X 1.8X 2X
100 2X 1.4X 1.5X
500 1.5X

 

[noelex]

The maximum current that a battery will deliver in a short circuit situation is difficult to predict. This maximum current will only be sustained for a very brief period of time (milliseconds or even fractions of milliseconds), but if the IC rating of the fuse is exceeded even for this brief period, the fuse can weld itself closed and then not offer any protection even when the current subsequently drops much lower.

You cannot look just at the current the fuse will supply for 0.1 seconds and deduce anything meaningful about the suitability of the IC rating.

There is debate if a single 280 Ahr lithium battery (and BMS) can deliver more than 10 kA in a short circuit situation, but given this uncertainty, it seems prudent to fit a class T or alternatively a NH fuse as these have IC ratings of 20 kA or greater.

 

[Daverw]

I have just completed install and had the same question, I eventually spoke to Eaton the fuse maker technical, I was going to use NH fuses but they were too big to fit, the said fir 12v systems why not use cube fuses (MRBF) as you have found has 10k break and when I look ABYC recommended as well. They say that the break current is good as the fuse element is on the side of the fuse so when blown no direct spark gap, They are able to go direct on battery terminals so space good, I’ve also just used them on Blue sea buss bar

 

[andrewAB]

Thanks both for the replies.
I will go with a 350 Amp T class.
And probably a BEP 778-T2S-400-B or BlueSea 8-25502 holder. Both are 225-400A with M10 studs.

I was concerned by online reports of T class blowing with capacitor inrush on inverters when connected on powerup. Renogy have confirmed 350A T class for use with their 3000W inverter that I have.

Thanks again
Andrew

 

[rogerthebodger]

It is said that a fuse protects the wiring, so I rated my battery fuse to 20-30 % greater than the battery cable current capacity which in my case was 700 Amps as I have 1200 Ah LA battery bank

 

[Sea Change]

I've just been thinking through this myself as I install a lithium system.
I've used a Blue Seas MRBF three-way unit to combine the two batteries, leaving the third fuse for any potential future upgrade. Then a class T after that. It's probably overkill but I thought it best to have both individual battery protection, and a main system fuse.

 

[Refueler]

It is said that a fuse protects the wiring, so I rated my battery fuse to 20-30 % greater than the battery cable current capacity which in my case was 700 Amps as I have 1200 Ah LA battery bank

I read the OP when it first came online and thought ... Holy Smoly !!

Me - I'm just a simple guy who thinks in terms of stopping the wiring from starting a fire ... and also what is max amps that my system expects to be able to 'ask for' ... then I fuse accordingly.

But then I sat back after reading the numbers being talked about and wondered what ???????

Thank you Roger - So I am not only one with that sort of mindset ....

 

[RogerJolly]

I read the OP when it first came online and thought ... Holy Smoly !!

Me - I'm just a simple guy who thinks in terms of stopping the wiring from starting a fire ... and also what is max amps that my system expects to be able to 'ask for' ... then I fuse accordingly.

But then I sat back after reading the numbers being talked about and wondered what ???????

Thank you Roger - So I am not only one with that sort of mindset ....

The interrupt rating (thousands of amps) is different to the 'blow at' rating (tens/hundreds of amps). Tried to get my head around it a while ago but ended up with more questions than answers.

Interrupt rating is something like 'the highest amount of current the fuse can safely interrupt without breaking or causing an arc'

But where did these thousands of amps come from though? Surely the fuse blew long ago at tens/hundreds of amps. Must be in a transitory spike immediately after the short circuit, before the fuse got hot enough to be blown to bits?

If it's unsafe to exceed the interrupt rating, what is the potential harm - does an arc strike that maintains a high current flow indefinitely? Is there a damaging explosion of metal vapour?

 

[noelex]

The simplest way to view the specifications is to think of the fuse rating (for example, 200A) as the minimum current that will blow the fuse. The IC rating (or interrupt capacity, for example, 10kA) is the maximum current that will blow the fuse.

If the current is greater than the fuse rating, it will eventually blow. Current greater than the IC (even extremely briefly) may damage the fuse by welding the elements together such that it can no longer offer any protection no matter what the subsequent current is.

The manufacturers list both ratings for most fuses that are likely to used on a boat.

Both numbers are important to design effective protection from short circuits.

 

[PaulRainbow]

It is said that a fuse protects the wiring, so I rated my battery fuse to 20-30 % greater than the battery cable current capacity which in my case was 700 Amps as I have 1200 Ah LA battery bank

What type of fuses did you use for the main battery fuses ?

 

[noelex]

If it's unsafe to exceed the interrupt rating, what is the potential harm - does an arc strike that maintains a high current flow indefinitely? Is there a damaging explosion of metal vapour?

If the IC rating of the fuse is exceeded, the fuse may no longer work at all. Batteries are only capable of delivering the very high currents which may exceed the IC rating for a very brief period of time. Milliseconds or less.

However, in a short circuit situation, a battery bank is capable of delivering a much lower current of perhaps many hundreds of amps for a much longer period of time. If the fuse has been damaged because its IC rating has been exceeded, it will not work as designed and may continue to conduct these high short circuit currents while the battery is capable of maintaining them.

The fundamental chemistry of lead-acid batteries limits the maximum short circuit currents to a level where ANL fuses have an adequate IC rating for even large marine battery banks. For small banks, fuses with a lower IC rating, such as MEGA fuses, are sometimes used.

The lower internal resistance of lithium cells creates the possibility of much higher short circuit currents. Ironically, the BMS units fitted to lithium batteries sometimes limit the continuous current delivery to a lower value than an equivalent lead-acid battery, but in a fault condition with a short circuit, there is the risk the battery bank can deliver a current that may exceed the IC rating of the main battery fuse, even when this same fuse was perfectly safe for a similar lead-acid battery system.

Consequently, if changing from a lead-acid battery bank to a lithium bank is important, consider if the IC rating of the main battery fuse is still adequate with the new chemistry. Most electrical engineers would recommend switching to a class T or NH fuse (with a 20KA+ rating) as the main battery fuse with a lithium marine battery bank, although some feel a MRBF fuse (with a 10kA rating) is reasonable for a small lithium battery.